Down peak traffic occurs in a building when a significant percentage of the people in the building all want to travel down to the lobby at the same time. This occurs often in office buildings at lunchtime or at the end of the work day when people are anxious to leave their offices and head home. It can also occur in residential buildings at the start of the day when people are leaving their homes to go to work, or in hotels when many people are heading to the lobby near check-out time.
Most multi-car elevator systems have an up peak mode which is relatively easy to handle, since all of the calls being handled as up-peak calls originate from the lobby floor. In contrast, during down peak, the calls originate on any floor of the building, and only terminate at a common floor.
Traditional means of handling down peak traffic in a building involves dispatching cars in such a way that empty cars travel to the highest floors with demand, answer the demand at successive floors and then carry those passengers to the lobby. What often happens, however, is that the elevator cars become full at the higher floors and are forced to bypass passengers waiting for service at lower floors. This can lead to unacceptably long waits for those at the lower floors.
There have been some attempts at down peak sectoring, i.e., dividing the hoistway into groups of floors and serving those groups in some type of ordered fashion to prevent any set of floors, such as the lower ones, from being starved of service. When cars only serve the demand in the sector that they are assigned to, however, it can lead to excess capacity in a car that is not being utilized. For example, if a car serves a sector that includes floors 5, 6 and 7 but there are only a few passengers waiting for service in that sector, it may travel all the way to the lobby without considering passengers on floors 2, 3 or 4 who have been waiting for service. If the demand for service on those floors is very heavy, then the car assigned to that sector may not be able to keep up with the demand.